Solar cell assembly with diffraction gratings

ABSTRACT

A solar cell structure using either a dye-sensitized or organic absorber is provided with a diffraction grating on at least one side to enhance the travel of first order diffraction components through the photo sensitive material. A two-sided cell uses diffraction gratings both top and bottom wherein the periodic diffraction elements of one grating are shifted by one-quarter of the grating period relative to the other.

FIELD OF THE INVENTION

This invention relates to solar cells and more particularly to solar cells of the dye-sensitized or organic absorber types using diffraction gratings to create oblique first order diffraction mode components which traverse the absorber thereby to enhance efficiency of the cell.

BACKGROUND OF THE INVENTION

Solar cells of various types have been developed for the purpose of converting unpolarized sunlight into electrical energy. Major objectives of solar cell research are increased efficiency and reduced production costs. Solar cells using dye-sensitized photoelectrodes and/or organic absorbers are substantially less expensive to manufacture than the conventional crystalline silicon solar cells.

BRIEF SUMMARY OF THE INVENTION

The first aspect of the present invention is a solar cell assembly of improved efficiency resulting from the combination of solar cell and diffraction grating technologies. As further described herein, the solar absorber may be of the type using dye-sensitized or organic absorbers and the diffraction grating may take the form of one or more substrates of glass or other optically transparent material bonded to one or both of the electrode layers on opposite sides of the photocell and exhibiting a pattern of diffraction grating material such as TiO₂ embedded in the surface of the transparent material at the electrode boundary. The grating is structured to couple only the first order diffraction component of normal incident light.

According to a second aspect of the invention, we have discovered that it is possible to advantageously combine solar cell technology with a bilateral diffraction grating technology as described in the co-pending application for U.S. patent Ser. No. 12/692,688 filed Jan. 25, 2010 by Hideo Iizuka and Nader Engheta, the complete disclosure of which is incorporated herein by reference.

In accordance with this aspect of our discovery, diffraction gratings using substrates with periodically arranged diffraction grating materials embedded therein are placed on opposite sides of a two-sided solar cell such that unpolarized light is incident on both such sides. The periodicities of the two gratings are the same but one grating is shifted by a fraction, preferably one-quarter, of the period relative to the grating materials in the other of the two gratings, thereby to prevent the escape of oblique diffraction components traveling through the absorber and returning those components for a second pass. This has been found to enhance the efficiency of dye-sensitized and organic absorber type photocells.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views and wherein:

FIG. 1 is a cross-sectional view of a first solar cell assembly employing the invention;

FIG. 2 is a cross-sectional diagram of a second solar cell assembly embodying the invention;

FIG. 3 is a cross-sectional diagram of still a third photocell assembly or a portion thereof embodying an aspect of the invention;

FIG. 4 is a diagram illustrating the enhancement effect to first order diffraction components; and

FIG. 5 is a graph of wavelength vs. path length enhancement effects in the solar cells described herein.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

Referring now to the drawings and particular to FIG. 1, there is shown a solar cell assembly 10 comprising a photoelectrolytic cell 12 containing a dye-sensitized absorber or photoelectrode 14 in an electrolyte 16. In this case, the terms “photoelectrode” and “absorber” are used interchangeably with “absorber” as the generic term. The top boundary of the cell 24 is defined by a positive electrode 18 and the bottom side of the cell is defined by a negative electrode 20. Resin seals 22 close the sides of the electrolyte cell volume. The electrodes 18, 20 may be fluorine-doped tin dioxide (SnO₂:F) and the photoelectrode may be dye-sensitized titanium dioxide (TiO₂).

A lower diffraction grating is defined by a glass substrate 26 having a periodic arrangement of rectangular grooves formed in the surface thereof which is bonded to and forms an optical boundary with the fluorine-doped tin dioxide electrode 20. The grooves are filled with titanium dioxide (TiO₂) to form diffraction elements 30. An optically transparent plastic may be substituted for glass in the fabrication of the substrate 26. The surface on which light is incident from below the structure 10 is modified by a shallow, low periodicity rectangular groove pattern 28 to provide an anti-reflection property.

A second diffraction grating made up of a glass substrate 32 with periodic titanium dioxide element 34 is bonded to the upper electrode 18 and the light incident upper surface 36 is modified to exhibit an anti-reflection grating-type surface 36.

As indicated in FIG. 1, the periodic elements 30 of the lower substrate 26 are shifted by one-quarter of the geometric spacing period of the elements 34 in the upper diffraction grating 32, thus to perform an optical blocking function as is more fully described in the aforesaid pending application Ser. No. 12/692,688.

It is apparent from the foregoing that each of the gratings in the assembly of FIG. 1 consists of a periodic structure including TiO₂ rectangular elements filling periodic grooves in glass at the interface of the glass with the fluorine-doped tin dioxide electrodes. Unpolarized sunlight through the glass is diffracted mainly into the oblique first order diffraction mode resulting in the enhancement of the path of travel of that diffraction component through the absorbing layer 14 of the titanium dioxide, dye-enhanced photoelectrode 14 immersed in the electrolyte 16. The advantage to the double-sided arrangement of FIG. 1 is the fact that, with the quarter prior shift between the gratings 30, 34, unpolarized sunlight from both sides experiences the path enhancement. Note that, unlike the device described in the aforementioned application Ser. No. 12/692,688, the shifted gratings 26, 32 here are fixed.

The fabrication process for the structure in FIG. 1 involves the creation of periodic grooves in the glass substrates 26, 32, the filling of the grooves with titanium dioxide and the bonding of the resulting structure to the fluorine-doped tin dioxide electrodes 18, 20. The grating period is an order of wavelength to achieve oblique diffraction whereas the metric periods of the anti-reflection surfaces 28, 36 are much smaller. Platinum particles 38 are bonded to the inside surface of the electrode 18; i.e., the surface which is within the interior of the cell.

The glass components and the electrolyte in the structure of FIG. 1 has a refractive index of 1.5 while the absorbing layer and the tin dioxide electrodes have a refractive index of about 2. The refractive index of titanium dioxide is 2.38. The overall design has the following specification: period (P) of grating elements is 0.84λ to 0.9λ; fill factor (R) is 0.34 to 0.44; height (H) of the grating elements is 0.59λ to 0.69λ. The refractive index of the diffraction gratings 30, 34 is larger than those of the absorbing layer 14, electrolyte 16, and electrodes 18 20. For more details, see our co-pending application Ser. No. 12/638,334 filed Dec. 15, 2009 the content of which is incorporated herein by reference. Where λ is 750 nm, the period P is 655 nm or 0.87λ, the edge width is 255 mm, the fill factor is 0.39 and the grating height is 480 nm or 0.64λ.

FIG. 4 illustrates how the grating couples only the first order component of normal incident light into the cell. Here the structure is identical to FIG. 1 except that the top glass layer 32′ has no grating elements. FIG. 5 illustrates the result of the optical path of the enhancement realized in the structure of 10 of FIG. 1. The vertical axis represents the effective path length relative to the thickness of the absorbing layer 14. The optical path is calculated in one round trip; i.e., bottom to top and back to bottom. As shown in FIG. 5, path length enhancement of 2 is achieved from 650 nm to 800 nm with an average enhancement value of 1.8 over the wavelength range from 450 nm to 800 nm.

Referring now to FIG. 2, a second configuration for a dye-sensitized solar assembly 40 is shown. A cell filled with electrolyte 42 has sealed edges 44 to define an electrolytic cell containing a dye-sensitized titanium dioxide photoelectrode 52. A negative electrode 46 made of fluorine-doped tin dioxide forms a partial boundary layer and an L-shaped positive electrode 48 separated from the dye-sensitized electrode 52 by a silicon dioxide separator 50 forms the rest of the lower boundary. The physical configuration of coplanar but laterally opposite positive and negative electrodes in the structure of FIG. 2 enhances the ability to serialize solar cells electrically in side-by-side physical relationship.

The structure of FIG. 2 further comprises a glass grating element 54 with titanium dioxide periodically arranged grating elements 56 therein and exhibiting finely grooved pattern 58 for anti-reflection properties.

The top side of the structure shown in FIG. 2 comprises an upper glass substrate 60 with a periodic arrangement of grating elements 62 as well as a reflection reducing top pattern structure 64. Again, the refractive index of the grating 54, 56 greater than those of the absorbing layer, electrolyte, and electrodes.

The electrode 48 may use as a material of construction In₂O₃:Sn+Pt or carbon. If carbon is used, the platinum element may be omitted.

The solar cell structure of FIG. 2 functions essentially as described above with reference to FIG. 1. FIG. 4 illustrates the enhancement of the first order diffraction component travel path through the photoelectric cell and infers the increased efficiency which is possible as a result of this structure. As also explained above, the two-sided nature of the structure affords enhancement with respect to incident unpolarized sunlight from each of two directions.

FIG. 3 illustrates a still further embodiment of the invention in the form of a grating enhanced solar cell structure 70 including an organic absorber layer 72 bonded to an indium tin oxide negative electrode 74 on one side and to an aluminum positive electrode 76 on the opposite side. Glass diffraction layer 78 with periodic titanium dioxide diffraction elements 80 is bonded to the indium tin oxide electrode 74 and preferably modified on the light incident surface 82 to exhibit the low metric period grating grooves 82. Enhancement of first order component travel path through the organic absorber 72 is also realized in the structure of FIG. 3.

Various modifications and additions to the invention will occur to persons skilled in the art. By way of example, the top glass 32 in a single-sided embodiment having no upper diffraction grating elements 34 can be replaced with a polymeric film or plate. The bottom glass layer 26 can also be replaced by a transparent polymeric plate. The typical thickness of the glass element 26, 32 is from 0.5 mm to 5 mm. The titanium dioxide in the absorber 14 can be replaced by ZnO or SnO₂. The titanium dioxide grating material can also be replaced with Ta₂O₅, ZrO₂, or Nb₂O₅, all of which have a refractive index greater than 2. In addition, the geometry of the refraction elements 30, 34, 56, 62 and 80 is not necessarily rectangular but may also be triangular or “blazed”. 

1. A solar cell assembly comprising: a cell containing a photoelectric absorber selected from the group consisting of a dye-sensitized photoelectrode and an organic absorber; a first electrode of a first polarity; a second electrode of a second polarity; and a diffraction grating for coupling a first order component of normal incident light into said absorber.
 2. A solar cell assembly as defined in claim 1 wherein the grating comprises a layer of optically transparent materials such as glass bonded to the electrode and a periodic pattern of grating material embedded in the transparent material in the surface thereof bounding said one electrode.
 3. A solar cell assembly as defined in claim 2 wherein the grating material is titanium dioxide (TiO₂).
 4. A solar cell assembly as defined in claim 1 wherein the first and second electrodes are made of fluorine-doped tin dioxide (SnO₂:F).
 5. A solar cell assembly as defined in claim 1 wherein the cell contains an electrolyte.
 6. A solar cell assembly as defined in claim 2 wherein the periodicity of said pattern is an order of wavelength to achieve oblique first order incident light diffraction.
 7. A solar cell assembly as defined in claim 2 wherein the surface of the glass upon which unpolarized light is first incident is modified to create an anti-reflection characteristic.
 8. A solar cell assembly as defined in claim 7 wherein the modification comprises a grating of substantially increased periodicity relative to the periodicity of the diffraction grating.
 9. A solar cell assembly as defined in claim 1 wherein the diffraction characteristic of the grating is such as to achieve optical path enhancement of about 2 in the wavelength range of 650 nm to 800 nm.
 10. A bilateral solar cell comprising: an electrolytic cell; an absorber in the cell; a first diffraction grating having periodically arranged surface elements on one side of the cell; a second diffraction grating having periodically arranged surface elements on the other side of the cell; the periodically arranged elements of one of the gratings is laterally shifted by a about one-quarter of the element period relative to the elements of the other grating such that first order components of diffracted unpolarized light are caused to traverse the photocell more than once.
 11. A solar cell assembly as defined in claim 10 wherein the absorber is a dye-sensitized photoelectrode.
 12. A solar cell as defined in claim 11 wherein the cell comprises an electrolyte. 